A manufacturer of capacitors, such as multi-layer ceramic capacitors (MLCC), uses a test system to determine the quality of a lot of product before the product is sold to a customer. The test system performs several tests that provide data on the capacitance, dissipation factor and insulation resistance (IR). The data can then be used to sort the parts by tolerance and find those parts that are defective. For example, for a given capacitance value, a capacitor has a minimum value for resistance below which a capacitor is unacceptable.
Tests are performed in sequence depending on individual manufacturer requirements. For example, a part can first undergo a capacitance and dissipation factor measurement at one station using a capacitance meter. After this, a leakage test can be performed.
Turning now to FIG. 1, a theoretical plot of voltage across a capacitor being tested versus time is illustrated, where at t0 the part is at zero volts. At t1, the part begins charging to a programmed voltage value. In this example, the part is changed with a constant-current source. At t2, the part has reached the programmed value. At t3, all measurements are complete and the part can begin discharging. At t4, the part is discharged to zero volts. FIG. 2 is a theoretical plot of the current through the capacitor being tested versus time. At t0 the part is at zero volts, and therefore has no current flowing through the part. At t1, the part begins charging. At t2, the part is charged so it no longer accepts current. This graph assumes an ideal capacitor and neglects parasitics, such as leakage current and dielectric absorption. At t3, the part begins discharging by reversal of the current from the constant-current source. Current flows in the reverse direction until the part reaches zero volts at t4.
A leakage test, also called an insulation resistance (IR) test, occurs at the end of a period of time at which the part is held at the programmed voltage. This so-called “soak time” extends from t2 to t3. Just before discharge at time t3, a measurement to obtain leakage current is taken. The current through the capacitor when a voltage is applied is the leakage current, and the insulation resistance is equal to the voltage divided by the leakage current. The leakage current can be compared to a maximum current based on the type of capacitor being measured and the tolerance of the manufacturer such that a capacitor passing a current above the maximum value would fail. In the example of FIGS. 1 and 2, the leakage current would be zero.